Conventional framed house floor construction consists of an upper decking membrane supported on a structural framework. A floor membrane typically consists of one or more layers of plywood, lumber or other sheeting material securely attached to the framing. The commonly used design load for residential floors is forty (40) pounds per square foot (40 PSF).
Floor framing consists of equally spaced parallel floor joists supported by a limited number of transverse supports such as beams, piers and walls. There are many types of floor joists, including solid lumber, light gauge steel "C" shaped channels and various types of manufactured floor joists. The weight of this type of floor ranges between 8 and 20 PSF for a design load of 40 PSF.
Insulation is included between the floor joists if any face of this floor is exposed to the elements. Sheathing such as drywall is added to the underside of the floor if it is exposed to living or storage area below this floor.
Assembling a framed floor on site is relatively slow, primarily because of the large numbers of components and fasteners needed. To ensure that the entire structure above and below the floor is able to withstand forces of wind or earthquake, all components must be securely attached to each other and suitably anchored to the surrounding structure. To laterally stabilize the floor joists, lateral bracing or blocking is commonly used between these floor joists. Lumber, in its many forms, is usually the least expensive flooring material, but subject to fire and termites.
Concrete is another commonly used element but a very heavy flooring medium. There are three types of concrete floor slabs, namely, slab on grade, suspended slabs and precast floor panels.
Slab on grade consists of preparing a level site, adding and compacting a suitable base coarse, excavating for concrete beams to be cast in the floor with the floor slab, providing perimeter forms to retain the fluid concrete, and installing the plumbing and electrical components to be cast in, through or under the floor, adding suitable reinforcing steel bars then pouring the concrete slab, taking care to ensure the finished floor is both level and smooth. This type of floor uses an excessive quantity of concrete. Subsequently changes and alterations are difficult. The weight of concrete for this type of floor with the base coarse and concrete beams under the slab is 90 to 100 PSF.
Suspended slabs are commonly used on multi-level and multi-family housing. The primary advantage of concrete is that it provides a suitable fire resistant barrier between floors. This on-site casting process involves building a temporary supporting form to retain this heavy concrete in its fluid state and then removing this temporary supporting form seven to fourteen days later, when this steel reinforced concrete floor has developed sufficient strength to be self supporting. The finished weight of this type of floor ranges between 50 and 150 pounds per square foot. This type of floor is slow to erect and expensive. Excessive time and labor is used to erect and dismantle temporary forming components.
Several types of precast concrete flooring systems have been developed to avoid this slow on site labor intensive process of assembling and dismantling a temporary mold to cast concrete on site.
One such precast floor panel is a concrete hollow core planks usually 48" or 24" wide, reinforced with steel tendons. The mechanical manufacturing process includes casting longitudinal voids in these panels to reduce the weight of these heavy concrete panels by a mere 20-25%. This type of floor requires an additional concrete topping material of at least 1" thick in order to obtain a level floor surface and interlock the panels together. The finished weight of this type of floor ranges between 70 and 100 PSF. The width of this particular type of precast floor panel is limited to 24" or 48" because of the excessive weight of a panel and the size and cost of the erection equipment. These narrow precast planks speed up construction considerably but still require site welding between the panels and the addition of connector bars between the planks and the wall structure and a layer of cementitious material is used to complete the floor or provide a level surface.
Another type of precast floor panel consists of casting only the lower half of the floor panel in widths up to 96". The initial manufacturing weight of this type of precast floor is approximately 30 PSF. This incomplete precast panel is assembled on site and then used to support the casting of the upper half of this floor. The finished weight of this type of floor ranges between 60 and 90 PSF. This precast method eliminates still more of the on-site work, but still results in a heavy floor.
All of these types of concrete floors rely on the use of steel bars cast in one element, projecting into or through or welded to the next element, and all of these suspended concrete floors do not include insulation.
Self leveling cements have been specifically developed to fill any depressions left in the initial concrete pour. Self leveling cements are also used on framed floor construction to eliminate floor joints, squeaking plywood floors as well as providing a suitable level surface for ceramic and vinyl floor tiling.
The primary considerations in designing floors of constant thickness are, one (1) the designated floor load, two (2) the length of the unsupported span and three (3) the maximum allowable deflection of the floor. The greater the span or load, the greater the depth of the floor. The maximum stresses or forces usually occur at mid-span and consist of tension force on the underside of the floor and compressive forces on the top side of the floor. These stresses are reversed at the points of support.
In concrete floor design, the ideal lower tension members are preferably steel and the ideal upper compression member is concrete. In each of the above types of concrete floors the concrete upper skin provides the necessary surfacing to the entire floor, however the remaining 80% or more of concrete below this skin does not contribute to the strength of the floor, and in fact adds unnecessary weight and cost to the floor. The steel in tension near the lower face of the floor occupies a very small area and needs only to be locally encased 1/2 of cementitious material as surface protection or fire protection. Another design consideration is the manner and sequence in which the structural elements of the floor are integrated with the surrounding structure. Still other considerations are making provision for instalation of plumbing & electrical elements within and through the floor.